The objective is to study the changes that occur in the hypothalamus, pituitary, gonads and reproductive tract of aging female and male rats, in order to elucidate the mechanisms responsible for loss of reproductive functions. Major emphasis will be placed on studies in female rats that have ceased to exhibit estrous cycles. The functional capacity of the hypothalamus will be measured by assaying LRF, turnover of dopamine, norepinephrine and serotonin, and ability to respond to stimuli and drugs that normally produce release of gonadotropins in young mature female rats. Pituitary release of LH, FSH and prolactin will be measured by radioimmunoassay (RIA) in the blood. Changes in ovarian function will be followed by gross and histological examination of the ovaries, by taking daily vaginal smears, and by measuring estrogen and progesterone by RIA in the blood. Attempts will be made to reinitiate estrous cycles and to induce pregnancies in old female rats that have ceased to cycle, principally by use of drugs to stimulate hypothalamic function.